Journal Club: Auditory sensory cells are a lot like neuron; they have extracellular basket proteins.

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Today's journal article

Kim SY, Jeon JC, Park B, Kim DE. Extracellular baskets in inner hair cells and perineuronal nets in auditory nerves: Changes in noise-induced hearing loss rats. 

Why I picked this article

This research investigates the presence and role of proteins related to the structure called "Perineural net", in auditory sensory cells (= auditory hair cells). 

I picked this journal because the title sounded interesting, or more so, since I was not familiar with the term "perineural nets". Upon looking up some literature about this term: 

  • The perineural net is a protein-based structure outside of the cell (i.e. extracellular matrix)
  • The perineural net looks like a "net" or web, and surrounds the cell body of neurons. 
  • The perineural net appears during development and is important for the adaptability of neurons. 
(ref: Fawcett, J.W., Oohashi, T. & Pizzorusso, T. Nat Rev Neurosci 20, 451–465 (2019).)

Fawcett, Oohashi, Pizzorusso (2019). Red color is the perineural net, and green is the cell body of the neuron. You can see how perineural net (red) surrounds the neuron. 

There are also "extracellular basket", which is also like protein-based net surrounding the synapse region where neurons make contact. Together, the extracellular basket and the perineural net represent an additional protein-based layer that surrounds the neuron and affects its function. 

Auditory hair cells, more specifically inner hair cells, make many synaptic connections with auditory neurons. Researchers of this study investigated the hypothesis that such an extracellular basket and perineural net may be important for inner hair cell homeostasis and protect inner hair cells from injury. 

Some of the research findings

Animal models:

  • Sprague–Dawley (SD) rats
  • 2-day-old rats were exposed to white noise (2–20 kHz, 115 dB SPL), 3 h per day until they are 21-day-old. 
  • Animals in noise-exposed group developed profound hearing loss by 1-month old. 

Perineural net and extracellular basket protein detection:

  • ACAN (rabbit multiclonal; Abcam; ab315486; 1:100) = "aggrecan"
  • BCAN (rabbit monoclonal; Abcam; ab285161; 1:1000) = "brevican"
  • NCAN (rabbit monoclonal; Abcam; ab277525; 1:50) = "neurocan"
  • HAPLN1 (goat polyclonal; R&D; AF2608; 1:2000) = "hyaluronan and proteoglycan link protein 1"
  • PV (mouse monoclonal; Merck; MAB1572; 1:500)
  • GAD67 (mouse monoclonal; Abcam; ab26116; 1:2000) = synapse marker
  • PSD95 (mouse monoclonal; DSHB; K28/43; 1:30) = synapse marker

Findings:

  • ACAN, BCAN and HAPLN1 were found in the extracellular basket near the inner hair cell synapses. 
  • The number of the extracellular baskets surrounding inner hair cells decreased in the noise-exposed group. 
  • ACAN and NCAN were found inside the cell (=cytoplasmic space) of the spiral ganglion neurons (=auditory neurons). 
  • In the auditory nerve, a few large ganglionic cells (LGCs) were observed; these cells had ACAN, BCAN and HAPLN1. 

Overall, this study reported the presence of some of the extracellular basket and perineural net proteins in auditory inner hair cells and spiral ganglion neurons, and how they seem to decrease when animals were exposed to noise. 

Haruna's takeaway

It was a very interesting paper to read and also to search for some review articles on the extracellular basket and the perineural net. I felt out of date on the latest neuroscience! The fact that these proteins are found near inner hair cells does suggest that inner hair cells, and more broadly, sensory cells (which are epithelial cells and not neurons), may have these structures. Maybe in sensory cells with synapses, these extracellular baskets and perineural nets play similar roles. It is a very small structure and needs to be shown to be outside the cell - so I wonder if a super-resolution imaging expert will follow up to show the "net" and "basket" with even more details and in 3D...! 

The relationship between the decrease in ACAN/NACAN and noise exposure remains to be investigated. It may simply be the secondary consequence of noise exposure, causing inner hair cells to lose synapses, or inner hair cells/surrounding cells becoming unhealthy. On this note, I wonder how these extracellular baskets and perineural nets are maintained; are they maintained by neurons or other neighbouring cells? The current literature seems to suggest a bit of both. 

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This is Haruna's 16/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.